Cognitive radios are devices, which are capable of adapting their functionality according to the surrounding radio environment, user needs, and/or other circumstances. For example, a cognitive radio device may receive request from a user to create communication link to another user. The device may then independently determine how the communication should be actually implemented for optimized connectivity, capacity and user experience. Cognitive radio systems generally employ dynamic spectrum use to achieve maximum flexibility. A cognitive radio device is therefore to perform spectrum sensing to determine whether certain frequency range is currently used or not.
Orthogonal frequency division multiplexing (OFDM) is a multicarrier modulation method, which has been widely used in modern communication systems. Examples of such systems are the digital broadcasting standards DAB (Digital Audio Broadcasting), DVB-T (Digital Video Broadcasting-Terrestrial), DVB-T2 (Digital Video Broadcasting-2nd generation terrestrial), DVB-H (Digital Video Broadcasting-Handheld), CMMB (Chinese Mobile Multimedia Broadcasting, ISDB-T (Integrated Services Digital Broadcasting-Terrestrial), and T-DMB (Terrestrial Digital Multimedia Broadcasting). OFDM has been also applied in wireless local and wide area networks (LAN/WAN) such as the IEEE 802.11 standards. OFDM is also the technology for the latest cellular communication system in the 3GPP Long Term Evolution.
An OFDM signal may comprise a plurality of subcarriers modulated by constellation symbols, i.e., complex numbers carrying the data. The modulated subcarriers form individual subchannels which can be in a transmitter bundled together by the inverse fast Fourier transform (IFFT), which translates the created frequency domain subchannels into a time domain signal. Similarly, a receiver may perform fast Fourier transform (FFT) to access the individual subchannels and to decode the transmitted data.
Although the present invention is described using OFDM as an example, it should not be interpreted as limiting the scope of the invention. Conversely, the present invention may be applied to any signals with suitable statistical properties.